Where boreholes are drilled so that an explosive charge may be delivered to an underground earth structure, it is a problem in that water located at the lower regions of the borehole may prevent or inhibit detonation of the explosive. In an attempt to overcome this problem, waterproof explosives are used, however, such waterproof explosives are generally expensive in comparison to other explosives.
Where more than one explosive deposit is to be placed at spaced intervals in a borehole, it has been conventional to apply a column of concrete or the like to a portion of the borehole so that a subsequent explosive charge may be spaced at a distance from the bottom of the borehole. The application of concrete or the like to this region of the borehole is both time consuming and expensive. The placing of plugs at selected depths in a borehole is generally called decking. Presently known decking methods are costly and inefficient.
One example of a decking is a wooden or concrete disk having a cross section substantially corresponding with the bore-cross section and lowered by rope to the water surface. It has also been proposed to use as a decking a polyester resin which floats on the water and solidifies after about 45 minutes. More recently, it has been suggested to replace the decking with a gel explosive slurry which floats on the water layer. Each of the foregoing methods suffers disadvantages. Timber or concrete deckings are not sufficiently reliable while gel explosive decking systems are costly and not easily prepared. Polyester resin is difficult to emplace without contamination of detonating cord, bore wall, and/or the resin and is slow to set. There thus remains a need for a decking system that is inexpensive, quick, convenient and which provides a reliable barrier above the water.
International patent application WO86/00660 describes a borehole plug composed of two or more co-reagents which expand when mixed separately contained such that they can be mixed when so required and further contain in an outer container adapted to retain the co-reagents during mixing and being dimensioned for dropping or lowering down a borehole. The co-reagents are described therein as the components of a polyurethane foam. The reaction between such co-reagents can be exothermic, and thus can present a hazard in the placement of explosives. Unless the co-reagents are carefully controlled, the temperatures achieved during the reaction between them can be such as to prematurely detonate explosives in contact with the borehole plug. This is especially so, where the borehole plug forms around detonating cord in the borehole.